This invention relates to packaging for a semiconductor device, and more particularly to a crack and delamination inhibiting lead frame for a semiconductor integrated circuit with a small window.
Conventional plastic semiconductor packages suffer from a failure mode referred to as xe2x80x9cpopcorn cracking.xe2x80x9d This failure mode occurs in packages that are exposed to ambient moisture and are then heated to high temperatures, typically during reflow soldering.
The problem apparently arises because plastic IC packages have a tendency to absorb moisture from the environment. The moisture diffuses into the encapsulant material and other materials such as the chip attach. During the solder reflow process, thermal vapor stresses developed at the chip attach/chip-pad interface or the encapsulant material/chip-pad interface cause delamination to occur, especially at areas of high interfacial stress.
A conventional full pad design is shown in FIGS. 6, 6A and 6B where a chip or die 10 has its lower side 12 secured to a chip pad 14 by a chip attach material 16. The chip attach 16 forms a fillet 18 between the side wall 20 of the chip 10 and the upper surface 22 of the chip pad 14 in an outer region known as the shoulder 24. The pad 14 and attached chip 10 are thereafter encapsulated in an encapsulant material 26, for example epoxy forming a package 30 in a known manner. In FIG. 6A, the package 30 is fabricated on a metal leadframe 31. The package 30 includes wire bonds 37.
The failure process appears to begin with delamination or cracking of the bond between the chip and the chip-pad. This delamination may be caused by differential expansion due to the differing coefficients of thermal expansion of adjacent materials within the package. Delamination 38 of the chip 10 from the chip pad 14 can occur when the interfacial stresses exceed the interfacial strength. Once delamination begins, it can propagate. The expanding void created by this delamination is invaded by water vapor, previously absorbed into the encapsulant material, and driven from the encapsulant by the rise in temperature. If the delamination covers a large area, the resulting long moment across which expansive water vapor forces act allows those forces to overcome the cohesive forces within the encapsulant material. In particular, delamination 38 often starts near the corner 32 of the chip 10, where the chip 10 meets the shoulder 24. In the case of the full pad design shown, delamination can rapidly propagate over the entire pad area. This can cause the package 30 to crack from the outer edge 34 of the chip pad 14 where the cohesive strength of the encapsulant material 26 is exceeded. The resulting crack may propagate through the encapsulant material 26 to the outer surface 36 of the package 30. In a like manner, delamination of the chip pad 14 from the encapsulant material 26 can also act as a crack source resulting in a popcorn failure.
These problems necessitate storage of components in humidity controlled environments prior to reflow soldering. Such required storage procedures represent additional cost and uncertainty in product quality.
The invention is based upon the discovery that a lead frame for a crack resistant integrated circuit package has an apertured frame, of reduced size, smaller than the integrated circuit. In a package utilizing the lead frame, the integrated circuit or chip is attached to the upper surface of the frame, and encapsulant material encloses and surrounds the frame and the chip. The encapsulant material bonds to a majority of the surface area of the chip and hardens to complete the package.
The invention provides a lead frame that reduces the initial adhesive failure, or delamination, that can occur during high temperature exposure that results in popcorn cracking. In an exemplary embodiment, this is achieved by reducing or minimizing the size of the attachment surface of the frame to the chip. The minimal attachment surface limits the propagation of cracks and increases the available bonding surface area below the chip and encapsulant.